eum = "a man/one", ut opinoreum repetendum, "as someone who should be recalled", I think
"While it indeed was successful, nevertheless I consider myself, who subsequently was in the greatest positions of power, as one [/a man] who, should he be[/were he] recalled as such [i.e., a quaestor], would not garner that much glory from praise due the quaestorship."

This is how I understand the sentence:
For although it was a success, I don't need to brag much about my quaestorship, since I showed what I was made of afterwards when I held the highest office (consul).

Ingrid's question is a lovely one. I imagined "eum repetendum" and "multum repetendum". I thought that "eum repetendum" made sense to me but if you say that's clearly wrong, I presume you mean "multum repetendum sit" is clearly right. That's fine.